Nickel-molybdenum alloys been known, for about sixty years, for use as wet corrosion-resistant articles, thermionic valves, high-temperature resistant articles and other engineering uses. These alloys have been available generally in the form of castings, wrought products, and as materials for use in welding and weld overlaying.
The best known commercial products are alloy B and alloy B-2 produced by Haynes International, Inc. under their registered trademark HASTELLOY. Alloy B nominally contains about 28% molybdenum, about 5% iron, about 0.30% vanadium, up to 2.5% cobalt, less than 1% silicon and the balance nickel plus impurities. Alloy B-2 nominally contains about 28% molybdenum, less than 2% iron, less than 1.0% cobalt, less than 0.1% silicon and the balance nickel plus impurities.
Also known in the art is HASTELLOY.RTM. alloy W which is especially suited for use as wellding wire for dissimilar alloys. Alloy W nominally contains about 24% molybdenum, about 5% iron, about 5% chromium, up to 0.60% vanadium and the balance nickel plus impurities.
U.S. Pat. Nos. 1,375,082 and 1,375,083 disclose nickel-molybdenum alloys, respectively, with and without manganese additions. The molybdenum content varies from a preferred 10% up to a maximum of 20%. U.S. Pat. No. 1,710,445 discloses an alloy system containing essentially molybdenum 15 to 40%, iron 10 to 40%, balance nickel plus modifying elements. U.S. Pat. No. 2,196,699 discloses a nickel-base alloy containing up to about 25% molybdenum plus antimony and other elements. U.S. Pat. No. 2,207,380 discloses a nickel-base alloy containing 18 to 40% molybdenum plus essential contents of manganese and silver. U.S. Pat. No. 2,315,497 discloses a nickel-molybdenum-iron alloy containing 10 to 40% molybdenum and 4 to 25% iron with critical lower limits of copper content. U.S. Pat. No. 2,109,285 discloses a 28-40% molybdenum nickel alloy with total silicon and carbon at less than 0.15% as impurities. U.S. Pat. Nos. 2,404,247 and 2,404,308 relate to nickel-base alloys containing 15 to 25% molybdenum plus required titanium, selenium and manganese.
The alloys of the prior art have found many valuable uses especially in exposure to wet corrosion conditions. Alloys containing nickel and molybdenum have been used also in the production of welding materials; for example: metal powders, cast rod, and welding wire. These alloys are especially useful as structural materials and weldments in the construction of vessels and plumbing handling acids, i.e., hot hydrochloric acid and the like. There is a constant need in the art for improved alloys of this class to reduce costs in long term use of articles and apparatus (such as vessels and plumbing) in a variety of acids and at higher temperatures.
As used herein the term, "structural material," shall mean a component of a manufactured article - for example, the hub as a component of a wheel. The term "weldment" shall mean "an assembly whose component parts are joined by welding."
In industrial use, there are many variables in acids and acid concentrations, temperatures and times at temperature and other factors that affect the alloy. Some of these factors may be oxidizing or reducing atmospheres, need for hardness or need for ductility, exposure to extreme heat or cold. Because of this, there can be no one perfectly ideal alloy for all industrial uses. There is a constant need in the art for improved alloys of this class, that provide a valuable combination of characteristics for many industrial uses.
The molybdenum rich nickel alloys of this class are especially resistant to reducing acids, i.e., hydrochloric and sulfuric acids.
The failure mechanism in fabricated components of these alloys appears most frequently in the weld and is attributable to the segregation of molybdenum. This results in the difference in molybdenum contents between dentrities and interdentritic regions. Increased dissolution and/or corrosion is found in the molybdenum lean phase.
Generally, it is expected that a homogenization heat treatment (such as anneal or cold work plus anneal) should reduce the tendency to such failures. However, experimental tests have shown that in some cases a homogenization treatment actually results in a higher corrosion rate in hydrochloric and sulfuric acids.
In a series of tests, prior art alloy B-2 was tested under several conditions in an effort to reduce such failures. Table 1 shows results of the testing. In Table 1, Condition A was as-welded (gas tungsten arc). Conditions B, C and D were as-welded plus 30% cold reduction. Finally, Condition B was annealed at 1066.degree. C. for about 15 minutes, then water quenched; Condition C was annealed at 1121.degree. C. for about 15 minutes, then water quenched; and Condition D was annealed at 1149.degree. C. for about 15 minutes, then water quenched.
The data in Table 1 clearly show a homogenization heat treatment after welding does not improve prior art alloy B-2, which contains about 28% molybdenum.